1. Field of the Invention
The present invention relates to a SAW filter that has an improved transmission characteristic and that is reduced in size, and a communication device including such a SAW filter.
2. Description of the Related Art
In recent years, as mobile communication apparatuses including an automobile phone, a mobile phone, and so forth, have been reduced in size and weight and configured to accommodate higher frequencies, small and lightweight SAW filters are often used as filters mounted on the above-described communication apparatuses. Further, the use of mobile-phone systems whose transmit frequency and receive frequency are close to each other has been increasing. Subsequently, when a SAW filter is used as a transmission filter, it becomes necessary to increase an attenuation of the receive-frequency band thereof. Further, where the SAW filter is used as a reception filter, it becomes necessary to increase an attenuation of the transmit-frequency band of the SAW filter. That is to say, the demand for increasing an attenuation near the pass band of the SAW filter has been increasing.
For satisfying the above-described demand, many patents disclosing technologies for increasing the attenuation near the pass band of a SAW filter have been filed. For example, see Patent document 1 (Japanese Unexamined Patent Application Publication No. 2000-196399 published on Jul. 14, 2000) and Patent document 2 (Japanese Unexamined Patent Application Publication No. 10-261935 published on Sep. 29, 1998). The Patent document 1 discloses a SAW filter 101 having interdigital transducers 102 to 104 provided between reflectors 105 and 106, as shown in FIG. 39. Since some of the electrode fingers of each of the reflectors 105 and 106 are interdigital-width weighted, a spurious response generated at the lower side of the pass band of the SAW filter 101 is reduced. Further, the attenuation of the SAW filter 101 increases.
According to the above-described technologies, however, it is necessary to increase the number of the electrode fingers having the interdigital-width weight of the reflectors for reducing the spurious generated near the lower side of the pass band, even though the size of the SAW filter increases, since the degree of change of frequency A whose reflectivity becomes zero shown in FIG. 40 is smaller than those of frequencies B to D whose reflectivity becomes zero, where the change occurs as the number of the electrode fingers of the reflectors changes. Therefore, the valley indicated by A of the reflectivity cannot be reduced even though the electrode fingers of the reflectors are arranged to have the interdigital-width weight.